This invention relates generally to synchronous electric machines, and more particularly, to a circuit for damping rotational oscillations resulting from hunting by synchronous electrical generators.
It is a characteristic of synchronous machines which are operated as motors or generators, that the spatial relationship between a magnetic field which rotates along a stator, and a magnetic field which rotates with a rotor, is a function of the load on the machine. For example, a lightly-loaded synchronous machine which is operating as a motor requires only a small electro-magnetic torque to maintain synchronism between the rotation of the rotor and the rotation of the stator field. If the mechanical loading of the motor shaft is increased, the rotor must retard in space phase so as to assume a spatial phase angle which approaches ninety degrees, at which angle maximum torque is produced. Since the rate of rotation of the magnetic field of the stator is constant for a given line frequency, the rotor must temporarily vary its speed so as to achieve a new spatial phase angle which corresponds to the desired torque output. This search by the rotor for a new space-phase position is called "hunting."
In voltage-regulated synchronous generators which are connected in parallel with transmission networks as loads, magnetic rotor wheel hunting can result from the effects of switching processes or other disturbances in the network. In addition to network disturbances, hunting in synchronous generators may be produced by the application of an uneven driving torque, such as that produced by a diesel engine, which may cause the position of the generator rotor periodically either to lead or lag with respect to the optimum space-phase orientation between the rotor and stator. Thus, hunting in synchronous electric machines may be produced by a variety of disturbances, affecting either the electrical or mechanical loads.
It is, therefore, an object of this invention to provide a circuit of reducing hunting in electric machines.